Pre-heating of metals prior to welding involves heating base metal pieces in the region surrounding a weld joint to a suitable pre-heat temperature. Pre-heating slows the cooling rate in both the weld metal and the base metal, which produces a more ductile metallurgical structure with greater resistance to cracking. Further, a slower cooling rate allows hydrogen, that may be present in the weld, to diffuse from the weld, which reduces the potential for cracking. Also, slower cooling rates reduce the shrinkage stresses in the weld in adjacent base metal.
After pre-heating is performed, additional heat is added during the welding process. After each weld bead is laid, the inter-pass temperature, which is the base metal temperature at the location where the next weld is to begin after the last bead has been laid, necessarily needs to be within a suitable pre-heat temperature range to produce desired results. Pre-heat temperature ranges may vary according to the base metals employed, as well as the thicknesses of these sections and types of welds to be made. Code requirements, in certain situations, require pre-heating prior to initiating the welding process. Hence, pre-heating can be an important factor in performing proper welding processes.